Many different apparatuses for balancing an out of balance condition in a rotating body are known. Such apparatuses generally include a counterweight having a weight of a predetermined value which is located at a predetermined position from the axis of rotation to oppose the imbalance in the rotating body. The magnitude of the imbalance is generally known and, accordingly, the necessary weight and position of the counterweight can be calculated so that the weight is positioned where it will act to counter the known imbalance. These apparatuses function satisfactorily for the purposes for which they are employed.
Under dynamic conditions; that is, when a body is rotating about an axis and an imbalance in the rotating body occurs because of external conditions or otherwise, the prior art is much less satisfactorily developed. For example, in a drill bit or in a drillstring, vibration induced forces during operation can create severe unbalances. One technique used to counteract such imbalances is disclosed in U.S. Pat. No. 4,905,776 (Beynet et al). Beynet et al teach a vibration dampening assembly with a plurality of annular grooves or races located about the periphery of the assembly and extending axially therealong. A plurality of balls or rollers are located in each of the races. Such balls or rollers are free to move along the races and thereby counteract the imbalance forces.
A further similar structure is disclosed in U.S. Pat. No. 4,674,356 (Kilgore). Kilgore teaches a plurality of balls freely movable in a race formed in an outer circumferential surface of the body which balls are used to counterbalance an imbalance in the rotating member.
There are, however, disadvantages in such prior art. Although the Beynet et al reference is satisfactory to remove large imbalances from the rotating body, it is difficult to utilise the teachings of Beynet et al where the length of the balancing apparatus is necessarily restricted which is often the case. Likewise, while the teachings of Beynet et al are satisfactory to generally remove large imbalances from the drillstring, there is no provision therein for removing all or most of the remaining imbalance thereafter, particularly the imbalance that may remain when the balls in the races of Beynet et al are located at their optimum positions in the races to counteract the imbalance.
This latter problem is also inherent in the above mentioned Kilgore reference. Kilgore teaches two counterbalance structures, one located at each end of a shaft, to offset the imbalance in the shaft or the unbalanced forces in the rotating structure which is movable with the shaft. If the balls are not located at their optimum positions, the imbalance in the shaft will not be removed.